vdso/datastore: Gate time data behind CONFIG_GENERIC_GETTIMEOFDAY

When the generic vDSO does not provide time functions, as for example on
riscv32, then the time data store is not necessary.

Avoid allocating these time data pages when not used.

Fixes: df7fcbef ("vdso: Add generic time data storage")
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250826-vdso-cleanups-v1-1-d9b65750e49f@linutronix.de